Hitherto, BH steel sheets (bake-hardenable steel sheets, hereinafter simply referred to as 340BH) of a TS: 340 MPa class has been applied to automobile exposure panels requiring dent resistance, such as hoods, doors, trunk lids, backdoors, or fenders. The 340BH is a ferrite single phase steel in which the amount of solid solution C is controlled by the addition of carbide/nitride formation elements, such as Nb or Ti, in an ultra-low carbon steel containing C: lower than 0.01% (% represents mass %, the same applies hereinafter), and solid solution strengthening is performed by Mn and P. In recent years, a need for reducing the car body weight has further increased. Then, investigations have been made to increase the strength of the exposure panels to which the 340BH has been applied, for a reduction in the thickness of the steel sheet, a reduction in the R/F (Reinforcement: inner reinforcement parts) with the same thickness, a reduction in the temperature and the time in a bake coating process, and the like.
However, when the strength of steel sheet is increased by adding a large amount of Mn and P to a conventional 340BH, the surface distortion of pressed parts remarkably occurs due to an increase in yield stress (YP). Here, the surface distortion refers to minute wrinkles and wave patterns of the press formed surface that are likely to occur in the outer circumferential surface of door knobs and the like. The surface distortion remarkably deteriorates the surface quality of automobiles. Thus, the steel sheets to be applied to the outer panels are required to have a low YP close to that of the current 340BH.
In order to increase the strength after press forming and bake coating while maintaining a low yield stress, the work hardening (WH) during press forming and bake hardening (BH) after press forming need to increase. In particular, in order to stably obtain high dent resistance without depending on the amount of plastic strain given during press forming, it is preferable to increase the BH. However, an increase in the BH causes deterioration of anti-aging properties. In particular, due to the recent globalization of vehicle manufacturing bases, the demand for the steel sheets for panels has been increasing not only in North America or Northeast Asia but in Southeast Asia, South America, India, and the like, and a further increase in the anti-aging properties has been demanded. For example, when the steel sheets are used in regions near the equator, the steel sheets are exposed to 40 to 50° C. for two to five months considering the transportation process or the storage period in warehouses on the regions. Thus, wrinkle-like patterns appears on the surface of pressed panel due to insufficient anti-aging properties in former ferrite single-phase steels. Thus, in recent years, the steel sheets are required to have more excellent anti-aging properties than those of former steel while maintaining a high BH as steel sheet properties.
Furthermore, the steel sheets for automobiles have been required to have excellent corrosion resistance. For example, in parts, such as doors, hoods, and trunk lids, flange portions of the exterior panels are bent by hem processing so as to be joined to the inner. Or, spot welding is performed. At the hem processed portions or the spot welded peripheral portions, the steel sheets are stuck to each other so that a chemical conversion coating is difficult to form during electrodeposition coating, and thus rust is likely to form. In particular, at corner portions in front of hoods or corner portions at door lower portions in which water is likely to collect and which are exposed to a humid atmosphere for a long period of time, holes are frequently formed due to rust. Thus, the steel sheets for exterior panels have been required to have excellent corrosion resistance. In particular, car body manufactures have been examining on an increase in antirust performance of car bodies for extending the hole formation resistance life to 12 years from 10 years (in former cases). Thus, it is indispensable for the steel sheets to have a sufficient corrosion resistance.
In view of such a background, PTL 1, for example, discloses a method for obtaining a galvannealed steel sheet having a low yield stress (YP) and high bake hardening (BH) by optimizing the cooling rate after annealing of a steel containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, and Cr: 0.023 to 0.8%, and forming a composite microstructure mainly containing ferrite and a martensite.
PTL 2 discloses a method for obtaining a galvanized steel sheet excellent in both bake hardening properties and room-temperature anti-aging properties by adding 0.02 to 1.5% of Mo to a steel containing C: more than 0.01% and lower than 0.03%, Mn: 0.5 to 2.5%, and B: 0.0025% or lower, and controlling the amount of sol.Al, N, B, and Mn in such a manner as to satisfy sol.Al≧9.7×N, B≧1.5×104×(Mn2+1) to thereby obtain a microstructure containing ferrite and a low-temperature transformation generation phase.
PTL 3 discloses a method for obtaining a steel sheet excellent in anti-aging properties by cooling a steel sheet containing C: 0.005% or more and lower than 0.04% and Mn: 0.5 to 3.0% to 650° C. or lower at a cooling rate of 70° C./s or more within 2 seconds after the termination of rolling in a hot-rolling process.
PTL 4 discloses a method for obtaining a steel sheet having a low yield ratio, a high BH, and excellent room-temperature anti-aging properties by adjusting Cr/Al to 30 or more in a steel containing C: 0.02 to 0.08%, Mn: 1.0 to 2.5%, P: 0.05% or lower, and Cr: more than 0.2% and 1.5% or lower.
PTL 5 discloses a method for obtaining a galvanized steel sheet having a high YP and a low BH by controlling Mn+1.29Cr to 2.1 to 2.8 in a steel containing C: 0.005 to 0.04%, Mn: 1.0 to 2.0%, and Cr: 0.2 to 1.0% and also adding a relatively large amount of Cr.
PTL 6 discloses a method for obtaining a steel sheet having excellent bake hardening properties by cooling a steel containing C: 0.01% or more and lower than 0.040%, Mn: 0.3 to 1.6%, Cr: 0.5% or lower, and Mo: 0.5% or lower to a temperature of 550 to 750° C. at cooling rate of 3 to 20° C./s after annealing, and then cooling to a temperature of 200° C. or lower at a cooling rate of 100° C./s or more.